Field of the Invention
The present invention concerns a method for operating a magnetic resonance apparatus in order to acquire magnetic resonance data with a prospective motion correction, as well as a magnetic resonance apparatus that is operable according to such a method.
Description of the Prior Art
Magnetic resonance signals or measured data are recorded (acquired) in magnetic resonance examinations, in order to generate spectra or images therefrom. When images are recorded, k-space, from which the images are reconstructed, can be filed line-by-line or point-by-point with the acquired (raw) data.
In some sequences and in optimum basic conditions it is possible to record so-called 4D data records. These are 3D data records that are repeatedly recorded in series. Such records are time-consuming to acquire, for which reason the acquisition of 4D data records with an acceptable time resolution represent a particular challenge. The images obtained in this way only show anatomical structures.
In contrast, magnetic resonance examinations for functional MRI as well as T1 maps intrinsically require at least a few minutes, since the T1 relaxation curve cannot be scanned any faster, even using the most efficient method.
A problem is that the examination object may exhibit motion or may move in this period of time. These motions result in motion artifacts in the reconstructed images.
Several methods are known for preventing motion artifacts in general.
Gating methods are known wherein the magnetic resonance examination is interrupted if the examination subject has moved out of a reference position. As soon as the subject is again at this position, the magnetic resonance examination is continued. In the case of periodic motions such as breathing and heartbeat this is easy to implement and can be triggered by an ECG. This method cannot be used, however, in the case of translatory motions, also called “bulk motions”, since in these the reference position sometimes cannot be achieved again. In addition, with this method it is not possible to keep the temporal repetition rate for image acquisition constant.
Furthermore it is known for the magnetic resonance signals to be corrected retrospectively, in other words after the recording. Several variants are possible in this category.
Firstly, markers can be securely attached to the patient and their motion can be plotted. Furthermore it is possible to record magnetic resonance signals known as navigator echoes. This type of retrospective correction of the recorded measured signals is known, for example, from U.S. Pat. No. 4,937,526 or U.S. Pat. No. 5,539,312.
Furthermore it is possible to correct the measured signals prospectively. In this case a continuous check is made during an examination to check whether the examination object has moved, in order to adjust the subsequent data recording to the motion, for example by adjusting the gradient settings. The advantage of this procedure is that the measured signals can be directly further processed after the recording. A prospective motion correction is known from Tisdall et al., MPRAGE Using EPI Navigators for Prospective Motion Correction, Proc. Intl. Soc. Magn. Reson. Med., 17, p. 4656, 2009.
In prospective motion correction, it is necessary for the navigator signals to be processed quickly. Hence rigid registration is preferred over elastic registration.
A problem with this is that incorrect registrations occur, especially when it is the head that is being examined. These become apparent as motion artifacts that are still present despite the motion correction.